The invention relates to an arrangement for measuring the radiation power of a laser, having a partially transmitting mirror arranged in the main beam of the laser, by which a portion of the radiation is coupled out, and having an electro optic transducer arranged in the partial beam.
In the case of high-powered lasers, particularly for surgical purposes, it is necessary to measure continuously the power coupled out of the laser resonator. For this purpose, it is known to arrange a partially transmitting mirror in the main beam of the laser by which a small percentage of the radiation power is coupled out by reflection on the mirror and is directed to a photodiode or the like (German Patent Document DE 32 05 507 C2). It was found, however, that, on the one hand, the reflection capacity of common splitter plates is different for light impinging in a parallel and perpendicularly polarized manner (relative to the plane of incidence) and, on the other hand, the orientation of the polarization plane changes during the pumping of laserable crystals, such as Nd-YAG, so that, as a function of the pumping effect, the proportion of the laser radiation impinging on the splitter plate in a perpendicularly or parallel polarized manner differs. In the case of a fixedly arranged splitter plate, specific proportions of the parallel and perpendicularly polarized light are coupled out. However, because of the above-mentioned conditions, the sum of both proportions is not proportional to the respective laser power.
It is therefore an object of the invention to provide an arrangement for measuring the radiation power of a laser which, over a wide measuring range, operates proportionally and independently of the privileged direction of the polarization plane of the laser radiation. This and other objects and advantages are achieved according to the invention, in which a second mirror, identical to the first in its characteristics, is placed in the path of the laser beam, with its angular position turned by 90.degree. with respect to the spacial orientation of the first mirror. With this arrangement the radiation power measured in the reflected beam of the second mirror is independent of the polarization condition of the main beam of the laser:
The intensity I of the main beam of the laser is additively composed of two components P and S, the polarization directions of which are oriented to the first mirror in parallel (P) with or perpendicular (S) to the plane of incidence. For both components, the first mirror has different reflection capacities R.sub.P1 and R.sub.S1. The intensity I.sub.1 of the first partial beam after the reflection on the first mirror will then amount to: EQU I.sub.1 =P.multidot.R.sub.P1 +S.multidot.R.sub.S1.
The components of the partial beam reflected on the second mirror are determined by the reflection coefficients R.sub.P2 and R.sub.S2. However, because of the 90.degree. rotation of the second mirror, the original P-component becomes the S-component, and vice versa. The following therefore applies to the intensity of the second partial beam after the reflection on the second mirror: EQU I.sub.2 =P.multidot.R.sub.P1 .multidot.R.sub.S2 +S.multidot.R.sub.S1 .multidot.R.sub.P2
Since R.sub.P1 =R.sub.P2 and R.sub.S1 =R.sub.S2, the following applies to the total twice repeated reflection: EQU R.sub.P1 .multidot.R.sub.S2 =R.sub.P2 .multidot.R.sub.S1 =R.sub.total =const. EQU I.sub.2 =R.sub.total (P+S).
The intensity I.sub.2 is therefore proportional to the intensity I of the main beam of the laser, and specifically independent of the distribution of the components P and S within the sum (P+S). Thus, the proportionality factor R.sub.total is constant.
Since complete independence from the polarization condition of the incident light will be achieved only if the two partially transmitting mirrors, have identical reflection coefficients for the respective polarization planes of the impinging light, it is expedient for the two mirrors to be made of a uniformly coated splitter plate.
When using light guiding systems transmitting laser radiation, such as fiber optics, it is also expedient to arrange the first partially transmitting mirror between the output of the laser resonator and the input to the light guiding system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.